I've alternated between ICE RIVER SPRINGS and BIG WIN Purified Water since they're the only brands I have stored as of right now. Tap water is too filled with chemicals to be drinkable.
Since this is the only thing you say you are consuming, it has to be suspect until you learn different.
The easiest way is to obtain some Distilled water that has also been filtered through a charcoal Carbon filter. This is the only way to get everything out leaving pure water.
If the pain goes away then you know.
Many bottled water has been shown to be no better than tap water.